The invention relates to the treatment of a substance with a treatment gas. The substance may be a liquid or may be in suspended form in a liquid, and in the following specification and claims the term "suspension" is intended to encompass both these forms of the substance. The invention is particularly applicable to a suspension of comminuted cellulosic fibrous material and utilizing an oxygen or oxygen containing gas for bleaching or delignification of the material.
Conventional commercial oxygen bleaching of paper pulp, utilizing apparatus such as disclosed in U.S. Pat. No. 3,660,225, normally takes place with the consistency of the pulp in the range of 20 to 30%. The degree of delignification (quantified by the KAPPA number of the pulp) is controlled mainly by varying the amount of sodium hydroxide (NaOH). The KAPPA number of the unbleached pulp is determined, and the addition of NaOH is varied with about 1 kilogram of NaOH per ton of air-dried pulp (ADMT) for each desired integer change of the KAPPA number. The temperature and the partial pressure of the oxygen is normally kept constant. During such high pulp concentration bleaching with oxygen an adequate quantity of oxygen is at all times maintained available as long as the pressure in the treatment vessel and the oxygen quantity in the reactor gas are kept about constant.
During conventional continuous oxygen bleaching of medium consistency pulp (e.g. in the range of 5 to 15% consistency) the available oxygen at all times is determined by the quantity of oxygen which is continuously added per ton of pulp. If the oxygen consumption increases due to higher washing loads in the preceding treatment stage, or due to a higher KAPPA number of the pulp, a lack of oxygen may appear at the end of the reaction cycle, which results in insufficient delignification (i.e., a KAPPA number that is too high) of the treated pulp. In order to maintain an effective control of the KAPPA number it is therefore important that the oxygen addition can be adjusted automatically during the treatment process, primarily to compensate for variations in washing losses and in the unbleached KAPPA number of the pulp.
According to the present invention a method is provided which provides for effective control of the oxygen addition so that a uniform delignification of the pulp occurs. While the invention is described primarily with respect to pulp treatment, it is to be understood that it is also applicable to other systems for the gas treatment of a suspension, and results in uniform treatment of the suspension in each case. The invention is based upon the observation that the pressure in a closed vessel filled with pulp in which oxygen is mixed-in decreases as the oxygen is consumed. Utilizing this observation, the gas consumption can be determined by measuring the density of the pulp at various stages, calculating the residual gas content of the pulp, calculating the gas consumption, and controlling the addition of gas based upon the gas consumption calculations.